A process has been disclosed which can make 20 micron or higher compression moulded PDMS membranes without using any special die for any particular thickness of membrane. PDMS and Dow mix is sandwiched between two surfactants applied foils. This foil sandwich is applied some pressure (2 Tons) by the press, so that sandwich becom flat. Now metal blocks are mounted on press and heatd at temp to 160degree C. Within five minutes this pressure is released and again applied. This cycle is repeatd for 5 times each after 5 minute's interval and then at least for 2 hrs at static temperature 160 degree C and pressure 15 tons static pressure till the room temperature comes. After these metal blocks are taken out and carefully foil sandwich is removed. Film is lifted with the help of knife-edge and water wash. The thickness of the film is measured using the thickness gauge.

Full Text

FIELD OF INVENTION
This invention relates to a process of making Poly Dimethyl Siloxane (PDMS) membranes of 20 micron thickness.
BACKGROUND OF THE INVENTION
Poly Dimethyl Siloxane (PDMS) material is known to posses higher diffusion coefficient and permeability, compared to various other polymers. The value of permeability for this material lies around 390x 10-8 cm2 S-1 atm-1 whereas the same in other polymers is its one tenth or less. Due to this, PDMS membranes find a place as gas separating membranes and being used in various instruments to permeate organic vapours and to suppress water vapours and NH3, NIX etc. The permeation property of this type of membrane has also been studied and reported in open literature.
The diffusion of flux or the permeated mass increases with the reduction in the thickness of membranes. Due to this, thin membranes having thickness around 25-microns of 1 Mil are used as barrier membrane in membrane inlet for selective permeation purposes in various instruments.
PDMS is silicon rubber material is being used, in industries as ideal gasket material, for use in high and low temperature, as this material sustains elastic property at around up to -30°C to +120°C without burning, cracking or oxidation. For making of gasket, washers etc, silicon rubber sheets used are made by compression moulding using a die for one particular thickness, with thickness ranging from 50-micron and higher up to few millimeters. PDMS membranes of lower thickness can be made by solution cast or using RTV compound but their mechanical properties are inferior.
US 5032279, granted to Lee, teaches a method of separation of fluids using PDMS membrane. However this patent does not disclose any technique to make 20-micron thin PDMS membranes specifically. Moreover patent does not describe about the compression moulded membrane, which do have better mechanical properties compared to solution case PDMS membrane. Use of silicon membranes for various types of sensors are also known in the state of art. However details of any simple process making compression moulded 20-micron or less membrane without using a die is not yet disclosed in either patents or in open literatures.

WE CLAIM
1. A process of making Poly Dimethyl Siloxane (PDMS) membranes of 20 micron thickness comprising the steps of
a. selecting two polished, smooth, circular or rectangular metal blocks;
b. selecting a smooth foil from any of mica, PET, Aluminium,
Polypropylene, SS sheet, thermally stable plastic or ceramic, cut in
circular, or rectangular form having same diameter like metal blocks;
c. cleaning the said foil by organic solvent;
d. applying surfactants on inner surface of the said foils so as to promote
transverse propagation of sandwiched material during moulding and to
inner surface of metal blocks and outer surface of said foils so as to
prevent them from sticking on heating;
e. sandwiching PDMS catalyst mixed Dow with the said foils in the form of
spherical ball at the center;
f. placing the said foil Dow sandwiches between smooth surfaces of the
said stated metal blocks in moulding press to make it flat by applying
pressure;
g. heating the metal blocks after taking out the said sandwich to bring it
to a stable temperature;
h. placing the said sandwich between the said metal blocks after the temperature has stabilized and applying pressure intermittently;
i. applying static pressure keeping the temperature constant to the metal block enclosing the sandwich for at least 15 minutes; and
j. cooling the said metal block enclosing the sandwich while keeping the pressure constant.
2. A process as claimed in claim 1, wherein the said organic solvent
selected from methanol, chloroform or isopropyl alcohol.
3. A process as claimed in claim 1, wherein the said surfactant selected from silicon oil, liquid soap, sodium lauryl sulfate or Tween-80.